Yes, in many Jain traditions you may cancel an uncertain/forgotten previous pachkhan and take a fresh pachkhan, but it should be done properly and sincerely, not casually.

A simple guideline:

• If you truly cannot remember whether it was porshi or saddhaporshi, and the earlier sankalp is unclear, then first mentally renounce the previous uncertainty and take a new, clear pachkhan.
• Take the new pachkhan before doing any activity of the day, and before getting out of bed if possible is very good for maintaining purity and discipline.
• When you take it again, be certain about the exact fast type you are taking.
• If you are in doubt, it is safer to follow the lesser assumption in observance only if your tradition or guru advises that, but the cleanest approach is to re-take it clearly with proper awareness.

In Jain practice, the main point is:

• sincerity
• clarity
• faithfulness to the vow
• avoiding negligence

So, yes, you can cancel the unclear one and take a new, clearly remembered pachkhan early in the morning, but do it with full awareness and respect.

For exact practice, different sangh and guru-parampara may have small differences.